The wear of cutting tools in a machining operation is a complex, dynamic process in which a number of operative mechanical and chemical factors contribute to degradation of the cutting edge. In steel roughing operations, in addition to gradual erosion of the tool edge (wear) an important consideration, due to severe loading conditions, is tool deformation and impact resistance.
The wear resistance of the tool is often increased by application of hard carbide, carbonitride or nitride coatings of metals such as titanium or oxides of aluminum. The wear resistance of coated tools is controlled by the properties of the substrate and coating. The impact resistance, as well as deformation resistance, of the tool stems from the bulk properties of the coated tool substrate. Substrate deformation may result in a complete or partial decohesion of the coating or its cracking and chipping, accelerating wear and leading to the destruction of the tool.
Prior art cites examples of attempts to improve tool impact resistance through the development of complex layered substrates having an outside layer of high deformability and an inside essentially less deformable core. The deformable surficial layer serves as a cushion which dissipates impact energy and renders a tool more impact resistant.
The prior art also suggested that impact resistance of cemented carbide tools containing excess carbon with a dense surficial layer essentially void of free carbon may be improved by application of multilayered coatings and through special diffusion treatment (U.S. Pat. No. 4,150,195).